A spark plug has a center electrode and a ground electrode kept insulated from each other by an insulator. There is a spark discharge gap defined between a front end portion of the center electrode and a distal end portion of the ground electrode. With the application of a voltage between the center electrode and the ground electrode, the spark plug generates a spark discharge within the spark discharge gap. Under the influence of such voltage application, however, a penetration breakage may occur in the insulator between the center electrode and the ground electrode. This results in the problem that the spark discharge cannot be properly generated within the spark discharge gap due to the flow of electric current through a broken site of the insulator.
In recent years, there is a tendency that the voltage applied to the spark plug increases with higher compression of fuel gas in internal combustion engines.
As the voltage applied to the spark plug increases, it becomes more likely that the penetration breakage will occur in the insulator of the spark plug. There has thus been a demand to establish techniques for preventing the occurrence of the penetration breakage in the insulator.
An advantage of the present invention is a spark plug capable of preventing a penetration breakage in an insulator.